(1). Here is a new little matte painting project. For this one I did some quick modeling/texturing etc in Maya and rendered everything with VRay to give me a decent 3D base. I also rendered some passes to help make the the paint-over process easier and faster. Never worked this way before but it was a looooot of fun. :)

Maya/VRay/Mudbox/WM/Photoshop/Nuke.

3D render/plate

(2). Also here is another quick matte painting I did for fun. I had been working a lot with 3D when I did this so wanted a little break from it.

The first image looks cool but I'm not sure that I buy the mid ground trees. Maybe real photographic elements might work better? Repeating what you did with other areas of the render. I think that the render looks cool with real warmth in the sunlight. Very idyllic. Maybe the matte could be more like that?

In your second shot the sky looks a little too dark to me. The shadow values in the background mountains are the same as the shadow values in the sky when I would expect the sky values to be brighter.
Making the mid ground mountains less symmetrical might help the shot. It's clear that the mountains surround an open space and help lead our eye to the larger mountain but the symmetry feels a little unnatural to me.
The left mid ground mountain is in shadow in your DMP. Maybe this is receiving some direct illumination? I find that creating very basic geometry in Maya and lighting that for reference can really help you to be sure that what you're painting is physically correct. If you can model something quickly this can really help. Just an idea.
Although we pass over a lot of ground I think it would be nice to see more parallax in the shot. The mountains are grouped very closely so it is tough to really see the parallax between them. Adding a foreground element which we fly over might help here to create more depth.

In your third shot the buildings are of a similar height. It isn't a problem however it might be nice to have more variation in their height. They are similar in design which may be what you want however I think having more variation in these buildings will help add interest and believability. I would also look at the window reflections in the plate and be sure that your DMP matches. The buildings also cut off sharply. Maybe you could add visible details to the roofs to break this?

In your fourth shot I am not sure on the camera move and its initial raise. I think that the city itself could be improved. Attempting to create a city in CG by oneself is a lot of work. The city as it stands in my opinion lacks realism. There is a lot of repetition and a lack of detail which I would expect to see. I would suggest based on your camera move to use photos to build your city or at least as a base to add to and then come in with specific architectural elements over the top. Towers etcetera. Organic elements such as trees may help too.

The foreground dunes lead the eye to the city but I'm not sure that they're really working. Maybe having a flat floor might be better and also more dynamic as the camera will be higher off the ground. I think that the fact that we don't see all the way to the bottom of the dunes where they meet the floor isn't really helping.